1. Field of the Invention
The present invention relates to a seed layer structure for a spin valve sensor and, more particularly, to a bilayer seed layer structure which increases a magnetoresistive coefficient of the spin valve sensor by improving its microstructure.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm that supports the slider above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The read head includes a sensor that is located between nonmagnetic electrically insulative first and second read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a nonmagnetic gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field into the pole pieces that fringes across the gap between the pole pieces at the ABS. The fringe field or the lack thereof writes information in tracks on moving media, such as in circular tracks on a rotating disk.
In recent read heads a spin valve sensor is employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer, and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to an air bearing surface (ABS) of the head and the magnetic moment of the free layer is located parallel to the ABS but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layers are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos .theta., where .theta. is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor resistance changes cause potential changes that are detected and processed as playback signals by the processing circuitry.
The spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. MR coefficient is dr/R were dr is the change in resistance of the spin valve sensor and R is the resistance of the spin valve sensor before the change. A spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve.
Another type of spin valve sensor is an antiparallel (AP) spin valve sensor. The AP pinned spin valve sensor differs from the simple spin valve sensor in that an AP pinned structure has multiple thin film layers instead of a single pinned layer. The AP pinned structure has an AP coupling layer sandwiched between first and second ferromagnetic pinned layers. The first pinned layer has its magnetic moment oriented in a first direction by exchange coupling to the antiferromagnetic pinning layer. The second pinned layer is immediately adjacent to the free layer and is antiparallel exchange coupled to the first pinned layer because of the minimal thickness (in the order of 8 .ANG.) of the AP coupling film between the first and second pinned layers. Accordingly, the magnetic moment of the second pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first pinned layer.
The AP pinned structure is preferred over the single pinned layer because the magnetic moments of the first and second pinned layers of the AP pinned structure subtractively combine to provide a net magnetic moment that is less than the magnetic moment of the single pinned layer. The direction of the net moment is determined by the thicker of the first and second pinned layers. A reduced net magnetic moment equates to a reduced demagnetization (demag) field from the AP pinned structure. Since the antiferromagnetic exchange coupling is inversely proportional to the net pinning moment, this increases exchange coupling between the first pinned layer and the pinning layer. The AP pinned spin valve sensor is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein.
In a bottom spin valve sensor the pinning layer is located at the bottom of the spin valve sensor. Layers of the spin valve sensor constructed on the pinning layer include a pinned layer structure which has its magnetic moment pinned by the pinning layer, the spacer layer and a free layer that has a magnetic moment that is free to rotate in response to a signal field. A typical material employed for the pinning layer in a bottom spin valve is nickel oxide (NiO). Another material in a first class of materials is alpha ferric oxide (.alpha. Fe.sub.2 O.sub.3).
A second class of materials, which may be employed for a pinning layer in a bottom spin valve, includes iridium manganese (IrMn), nickel manganese (NiMn), platinum manganese (PtMn) and iron manganese (FeMn) which are metals. An advantage of the second class of materials is that the pinning layer can be made thinner. A typical thickness of a nickel oxide (NiO) pinning layer, in the first class of materials, is 425 .ANG. while a typical thickness of an iridium manganese (IrMn) pinning layer, in the second class of materials, is 80 .ANG.. This is a difference of 345 .ANG.. Since it is desirable to keep the read gap (distance between the first and second shield layers) as thin as possible for promoting linear density of the head, iridium manganese (IrMn) is very desirable for use as a pinning layer. One of the disadvantages of the second class of materials, however, is that a read head employing a bottom pinning layer made of one of the second class of materials has a lower magnetoresistive coefficient (dr/R) than a read head having a pinning layer made from one of the first class of materials. For instance, in an antiparallel pinned spin valve sensor that employs a pinning layer made of nickel oxide (NiO), the magnetoresistive coefficient is approximately 7%, whereas an antiparallel pinned spin valve sensor employing a pinning layer made from iridium manganese (IrMn) has a magnetoresistive coefficient (dr/R) of approximately 4.5%. In both instances the pinning layer interfaced an aluminum oxide (Al.sub.2 O.sub.3) first read gap layer. In a simple spin valve sensor, when a single iridium manganese (IrMn) pinning layer was directly on an aluminum oxide (Al.sub.2 O.sub.3) first read gap layer, the magnetoresistive coefficient (dr/R) was approximately 4.0%. While iridium manganese (IrMn) has the advantage of less thickness than the first class of materials, it has a significant disadvantage in the loss of magnetoresistive coefficient (dr/R) when employed as a pinning layer. When the magnetoresistive coefficient (dr/R) is low the strength of the read signal is low, which equates to decreased storage capacity of a magnetic disk drive.